Why Small Engines Will Not Start in Spring

A machine that ran perfectly in October will not start in April. It has not broken. In the overwhelming majority of cases it has been poisoned by its own fuel.

This is the most common small engine failure, it is entirely preventable, and the prevention takes about ten minutes per machine.

What happens to petrol in storage

Petrol is not a stable substance. Left standing, the lighter fractions evaporate and what remains oxidises into heavier compounds — the sticky, varnish-like residue you find in a neglected carburettor. Fuel containing ethanol adds a second problem: ethanol is hygroscopic, meaning it attracts water out of the air. Given enough time and a vented tank, the fuel can separate, leaving a layer of water and ethanol at the bottom, which is exactly where the fuel pickup sits.

Inside a small carburettor the passages this has to flow through are extremely fine — the main jet on a mower is a hole you can barely see. It does not take much varnish to block one completely, and once blocked, no amount of pulling the cord will help.

The engine itself is usually fine. That is what makes this failure so frustrating: a perfectly good machine, immobilised by the contents of a part that costs very little.

The two methods that work

There are exactly two reliable approaches, and what causes trouble is doing neither.

Run it dry. Turn off the fuel tap if there is one and let the engine run until it stops of its own accord, then restart it briefly to clear what remains. This leaves the carburettor essentially empty and is the better option for anything you will not touch for months — a mower over winter, a strimmer, a chipper.

Treat the fuel and leave the tank full. Add a fuel stabiliser at the dose on the bottle, then run the engine for a few minutes so treated fuel actually reaches the carburettor. Leave the tank full rather than part-full, because an air space is where condensation forms. This is the right approach for anything you might need at short notice — above all a generator, which is useless if it needs an hour of work before it will start.

What does not work is draining the tank but leaving the carburettor full, which is what happens if you siphon the tank and walk away. The float bowl still holds fuel, and that is the fuel that turns to varnish.

The other three storage jobs

Fuel is the main event, but three more things are worth doing at the same time.

Change the oil before storage, not after. Used oil contains combustion acids and moisture. Left sitting in a crankcase for six months those attack bearing surfaces; fresh oil sitting there does no harm. This is counter-intuitive — it feels wasteful to change oil on a machine you are about to put away — but it is the right way round.

Clean the machine properly. Packed grass under a mower deck holds moisture against metal all winter. Sawdust and chain oil do the same to a saw. Ten minutes with a scraper is worth more than it looks.

Take batteries out and keep them charged. A lead-acid battery left discharged over a cold winter will very often not recover. Somewhere frost-free, on a maintainer or given an occasional top-up charge, and it will.

Two-stroke fuel is worse

If you run two-stroke machines — most strimmers, many chainsaws, some blowers — the problem is sharper. Mixed fuel degrades faster than straight petrol, and the oil can separate out of suspension over time. A separated mix run through an engine can mean inadequate lubrication at exactly the moment it matters.

The practical rule is to mix what you will use and use what you mix. Where that is not possible, pre-mixed fuel sold in sealed cans has a much longer stated shelf life than fuel you mix yourself, which is why people who use a saw only occasionally often find it worth the cost.

Doing it as one pass

The reason machines get missed is that winterising happens machine by machine, on different weekends, until you lose track of which ones you did. Doing all of them in one session against a list is both faster and more reliable, which is what the Outdoor Machinery Winterising Checklist is for — one row per machine, one column per job.

Then, when a machine does come out in spring and runs first time, record it. Sheets like the Lawn Mower Service Log and the Small Engine Service Log exist so that next autumn you can see what you did last autumn, and whether it worked.

If it is already too late

A machine that will not start after storage is usually recoverable. Fresh fuel is the first thing to try, then a new spark plug, then cleaning the carburettor — on most small engines the float bowl comes off with a single bolt and the main jet can be cleared with carburettor cleaner.

It is a genuinely approachable job on a mower or a strimmer and the parts are inexpensive. But it is an hour you did not need to spend, and next autumn you can avoid it entirely.